Semiconductor films, such as silicon films, are known to be used for providing pixels for liquid crystal display devices and organic light emitting diode displays. Such films have previously been processed (i.e., irradiated by an excimer laser and then crystallized) via excimer laser annealing (“ELA”) methods. However, the semiconductor films processed using such known ELA methods often suffer from microstructural non-uniformities such as edge effects, which manifest themselves in availing a non-uniform performance of thin-film transistor (“TFT”) devices fabricated on such films.
Such non-uniformity in edge regions is particularly problematic in that the visual transitions between the neighboring pixels corresponding to the irradiated and crystallized areas of the semiconductor thin film on the liquid crystal displays (“LCDs”) or organic light emitting diode displays are not as smooth as could be desired, and may even be visible in certain cases, which is undesired. This is also because the edge effects promote a low performance in TFT devices whose active regions are provided thereon.
Significant efforts have been made into the refinement of “conventional” ELA (also known as line-beam ELA) processes in the attempt to reduce or eliminate non-uniformities on the crystallized areas of the semiconductor thin film. For example, U.S. Pat. No. 5,766,989 issued to Maegawa et al., the entire disclosure of which is incorporated herein in its entirety by reference, describes the ELA methods for forming polycrystalline thin film and a method for fabricating a thin-film transistor. This publication attempts to address the problem of non-uniformity of characteristics across the substrate, and provide certain options for apparently suppressing such non-uniformities. However, details of previous approaches make it impossible to completely eliminate the non-uniformities that are introduced from the edge areas (which are typically between 100 μm to 1,000 μm or higher). Thus, the cross sectional area of the portions of the semiconductor thin film on which the TFT devices could be placed would be significantly reduced due to such disadvantageous edge effects causing large non-uniform edge areas which border these portions.
For example, one such conventional ELA process uses a long and narrow shaped beam 800 as shown in FIGS. 11A and 11B. The fluence of this beam 800 is above a melting level in a center portion 810 thereof, while side areas 820 of this beam 800 have a fluence that is gradually reduced at the edges thereof. The width of the center portion 810 of the beam 800 may be 1 cm and the length thereof may be 30 cm. In this manner, the beam can potentially irradiate the entire semiconductor thin film during one pass across it. As shown in FIG. 11B, portions 830 of the side areas 820 of the beam 800 can be provided between the melting level and the crystallization threshold. Thus, when the beam 800 irradiates particular portions of the semiconductor thin film which is then crystallized, such portions would likely have an edge regions in thus irradiated and crystallized areas of the semiconductor thin film that are likely to have large non-uniform edge regions spatially corresponding to the portions 830 of the beam 800. The edge regions are known to be disadvantageous for placing the TFT devices (and especially their active regions) thereon.
Attempts have been made to eliminate the edge effect (i.e., non-uniformity of the edge areas) of the irradiated and crystallized portions of the semiconductor film. U.S. Pat. No. 5,591,668 issued to Maegawa et al. tries to minimize these edge areas by using a substantially square shaped beams (rotated 45° and having rounded peaks) that sequentially overlap one another using a laser annealing method. However, such conventional procedure would require multiple irradiation by the beam pulses of the same areas, and the processing of the semiconductor film would be somewhat slow.
Accordingly, it is preferable to irradiate and crystallize at least some of the areas of the semiconductor films by passing the beam pulses through a mask in a way so as to eliminate the problem caused by such edge effect by clearly defining the profile of the beam pulse. It is preferable to significantly reduce the spatial scale associated with the edge region so as to make it possible to have such regions be provided away from the active regions of the TFT devices. In addition, multiple irradiations of the same area on the semiconductor film would therefore no longer be necessary.